A typical automotive-type solid electrolyte exhaust gas oxygen sensor is disclosed in U.S. Pat. No. 3,844,920 Burgett et al. It has a zirconia sensing element shaped as a tapered thimble. One end is open and has a circumferential flange. The other end is closed, and forms the most active part of the element. The interior and exterior of the thimble have separate porous electrode coatings of platinum or the like. The inner electrode is exposed to a source of oxygen, such as air or a mixed metal oxide, for establishing a reference potential. This electrode has generally been formed by painting a coating of a platinum ink onto the zirconia thimble, drying the coating, and then firing the coated thimble at an elevated temperature. An improved technique by which it can be applied is described and claimed in United States patent application Ser. No. 089,264 entitled "Reference Electrode Process for Exhaust Gas Oxygen Sensor", filed on Oct. 1, 1979 in the name of John Trevorrow and assigned to the assignee of this invention.
The outer electrode is exposed to the exhaust gas for establishing a potential determined by exhaust gas oxygen concentration. The outer electrode can be a porous thick film of platinum, like the inner electrode. However, it is preferred that this outer electrode be a thin film, applied by evaporation, sputtering, chemical vapor deposition or other such thin film deposition techniques. On the other hand, it has been difficult to consistently reproduce desirable properties, such as porosity and electrical parameters, in the thin film electrodes. As a result, yields of satisfactory electrode properties have been limited, and various ancillary procedures have been developed to improve them. For example, U.S. Pat. No. 3,978,006 Topp et al discloses heating the solid electrolyte body after electrode deposition, to form pores in the electrode coating if it is not porous as deposited. U.S. Pat. No. 4,136,000 Davis et al discloses treating the electroded sensor element chemically and electrolytically to enhance sensor properties. Moreover, it is known that zirconia-type exhaust gas sensors, particularly those with a sputtered exhaust gas electrode, are likely to change electrical characteristics after a short time in operation. Generally, there is an improvement, such as a reduction in switching response time. Consequently, it has been proposed to operate such sensors functionally in an actual or simulated exhaust gas stream until they are sufficiently stabilized, before installing them in an actual working system. Such treatments, of course, add to the cost of manufacture. Moreover, the yield of higher performance sensors is still inherently limited by the quality of the electrode film originally deposited. We have found how to sputter platinum films onto the zirconia surface in such a manner that the film is consistently porous as deposited and has a consistently high surface area as deposited, which contributes to a greater yield of high quality sensors. Sensors with low lean-to-rich switching response times are produced, without post-electroding treatments. Rich-to-lean switching response times are initially not nearly as low as lean-to-rich switching response times. However, they are generally readily reduced to acceptably low levels after only a short actual or simulated aging. Hence, a high yield of significantly fast sensors is obtained from only minimal actual aging. In fact, sensors with exhaust electrodes produced in accordance with this invention are susceptible to aging by a simple furnace treatment, as is disclosed and claimed in United States patent application Ser. No. 030,747 entitled "Aging Treatment for Exhaust Gas Oxygen Sensor", filed on Apr. 17, 1979 herewith in the names of Morris Berg, Slater W. Hawes, Frederick L. Kennard, III and Paul C. Kikuchi and assigned to the assignee hereof. In addition, a sizeable proportion of sensors having exhaust electrodes produced with this invention do not even need any post-electroding treatments for activation or stabilization. Their electrical properties as formed are more than adequate and remain substantially stable during initial sensor use.